Clear one saved pref (avatar pick, layout, widget toggle) — session-only.

REMOVES the row rather than storing a null: "no row" means "no choice made", which is what the avatar store reads to fall back to the tag identicon; a stored null would keep overriding it. Idempotent and never a 404 — a pref that was never set is not an error, so the answer says so: {"deleted": key, "existed": false}. A malformed key is still a 400, exactly like GET/PUT.